This invention relates to a method of controlling a spool-valve type, electromagnetic proportional pressure control valve used for accurate control of brake fluid pressure or the like.
Electromagnetic proportional pressure control valves have a driving means comprising an electromagnet and a spring. The driving means moves a spool to a position corresponding to its driving force, thereby controlling the fluid pressure applied to a load to a valve corresponding to the driving force of the driving means. A typical electromagnetic proportional pressure control valve is shown in FIG. 2.
The electromagnetic proportional pressure control valve 1 of FIG. 2 comprises a housing 2, a spool 9, a reaction pin 12 inserted at one end of the spool 9, a spring 13 for biasing the spool, and an electromagnet 14 for biasing, i.e. pulling, the spool 9 in the direction opposite to the direction in which the spool is biased by the spring 13.
The housing 2 has a spool guide bore 3 in which is substantially liquid-tightly and slidably received the spool 9, a supply port 4, an output port 5 and an exhaust port 6 all opening to the bore 3 at its intermediate portions, and first and second fluid chambers 7, 8 into which one and the other ends of the spool 9 protrude, respectively.
The spool 9 has a surface passage 10, and an internal passage 11 kept in communication with the load port 5. The internal passage 11 has one end open to the first fluid chamber 7, and at this end, the reaction pin 12 is substantially liquid-tightly inserted in the passage 11. Thus, a difference equal to the sectional area of the reaction pin is created between the area for bearing fluid pressure that urges the spool 9 toward the first fluid chamber 7 and the area for bearing fluid pressure that urges the spool toward the second fluid chamber 8. The spool 9 is thus biased downward in the figure under a thrust which is equal to the abovesaid difference in area multiplied by the pressure at the load port 5.
Between a land 9a formed on the outer periphery of the spool 9 and the supply port 4, a first valve portion 15 is formed to open and shut off communication between the supply port 4 and the output port 5. Between a land 9b formed on the outer periphery of the spool 9 and the exhaust port 6, a second valve portion 16 is formed to open and shut off communication between the output port 5 and the exhaust port 6. The degree of opening of each of the first and second valves 15, 16 changes with the spool position.
With this electromagnetic proportional pressure control valve 1, during a non-control state in which no current is supplied to the electromagnet 14, the spool 9 is maintained in the illustrated position by the spring 13. In this state, the first valve portion 15 is open, so that fluid pressure from the pressurizing port 4 flows into the output port 5.
When the electromagnet 14 is energized, the spool 9 is pulled downward in the figure by the electromagnetic force until the upward force balances with the downward force.
The relation at the balancing point is given by the following formula (1). Until the first valve portion 15 closes, the pressure at the output port 5 and the spool moving distance increase with the exciting current I. When the current I further increases after the first valve portion 15 has been closed, the second valve portion 16 will open, thus opening the output port 5 to the depressurizing port 6. The pressure at the output port 5 thus drops. EQU Fpr+Fsol=Fsp (1)
Fsp: force of the spring 13 PA1 Fsol: driving force by the electromagnet 14 PA1 Fpr: thrust resulting from fluid pressure PA1 1) In an arrangement in which the spool is vibrated by a dither current superimposed on the current supplied to the electromagnet, the amplitude or frequency of the dither current is changed so that the lower the temperature, the larger the amplitude and the lower the frequency. PA1 2) In an arrangement in which the spool is vibrated by a pulsating component of a PWM-controlled current, the frequency of pulse width modulation is changed so that the lower the temperature, the lower the frequency.
Fpr in the above equation is given by: EQU (P2-P3).multidot.S
Wherein P2 is the pressure at the output port 5 (load pressure), P3 is the reservoir pressure, and S is the sectional area of the reaction pin 12.
On the other hand, Fsol equals a-b.multidot.I.sub.2 (a and b are constants). Thus, the following relations are met: EQU (P2-P3).multidot.S+(a-b.multidot.I.sub.2)=Fsp EQU .thrfore.P2=(Fsp-a+b.multidot.I.sub.2)/S+P3 (2)
Since Fsp, a, b, S and P3 are all constants, the pressure P2 is proportional to the current I. In the equation (1), (Fsp-Fsol) is the spool driving force by the driving means.
FIG. 1 shows an automotive brake system including this electromagnetic proportional pressure control valve. In this system, the supply port 4 is connected to a fluid pressure source (pump 24), the output port 5 is connected to a wheel brake 22, and the exhaust port 6 and the first and second fluid chambers 7, 8 are connected to a reservoir 25. The valve 1 controls fluid pressure supplied to the wheel brake 22 for antilock control or other vehicle behavior control.
The illustrated brake system includes a shutoff valve 23 disposed between the master cylinder 21 and the wheel brake 22. During a normal control mode, the valve 23 is kept open by the pressure of an accumulator 26. When necessary, the electromagnetic proportional pressure control valve 1 indirectly controls the pressure in the wheel brake 22 by controlling the fluid supply pressure to the shutoff valve 23.
When the spool begins to move, a large driving force is needed to overcome the static frictional force. Thus, with an electromagnetic proportional pressure control valve including a spool, during a short period immediately after the spool has begun to move, the static frictional force tends to break a proportional relationship between the current supplied to the electromagnet and the controlled fluid pressure.
Thus, in order to prevent the static frictional force from acting on the spool, a dither (that is, force having a controlled amplitude and frequency) is applied to the spool to keep the spool vibrating with such a small amplitude that will not affect the fluid pressure on the load.
Such a dither is produced by a current created by superimposing an alternating current (which will be referred to as dither current) on the current supplied to the electromagnet so that the center of amplitude of the former will be on the latter current.
In vibrating the spool by applying a dither, if the current supplied to the electromagnet is in the region smaller than I.sub.1 (see FIG. 8) or in the region where magnetic saturation occurs in the magnetic circuit of the electromagnet, i.e. the region larger than I.sub.2, the dither effect (that is, spool vibrating power) tends to weaken due to decreased ratio of the solenoid force Fsol to the current I supplied (Fsol/I).
If the amplitude of the dither current is increased so that a sufficient dither effect is achievable in these regions, the dither effect in other regions will be too strong, that is, the spool tends to vibrate excessively.
An object of this invention is to provide a method of controlling an electromagnetic proportional pressure control valve which can substantially level the dither effect over the entire control region.